Aasimar are included as a playable race in Volo's Guide to Monsters. That they're not playable in BG3 doesn't mean they don't exist; they're like gnomes or dragonborn in that regard.
AFAIK Larian's said the races will be those listed in PHB. I'd like they to be in (just to see how Shadowheart reacts vs how she reacts to drow as much as anything) but if they put all the PHB I expect the workload would be too much and it would take resources from another aspect of the game.
